Can hackers not put garbage data into it to make the vaccines look more dangerous?
Of course - it's a totally open database that anyone can add to, whenever they want. Literally anyone could create fake entries such as those who wish to scare Americans off from getting the vaccine. That could include state actors (Russia, China, etc) that want to cause America further harm, or even passionate anti-vaxxers.
There's of course no way to tell which VAERS records are true and which aren't, but the strong likelihood is that there are at least "some" VAERS records of dubious origin..
COVIDiots sure love their clot-shots. Sad.